1. Field of the Invention
The present invention relates to a method and apparatus for handling PDCCH orders in a wireless communication system, and more particularly, to a method and apparatus capable of enhancing transmission efficiency and spectrum aggregation.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in base stations (Node Bs) alone rather than in Node Bs and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In the LTE system, in order to ensure uplink synchronization and normal data transmission, a network terminal sends a Physical Downlink Control Channel (PDCCH) order to a user equipment (UE), such that the UE triggers a Random Access (RA) procedure, and sends a packet including a dedicated preamble indicated in the PDCCH to the network terminal, to perform uplink synchronization.
Therefore, the network terminal can timely indicate the UE to trigger the RA procedure through the PDCCH order, to ensure uplink synchronization. However, to meet future requirements of all kinds of communication services, the 3rd Generation Partnership Project (3GPP) has started to work out a next generation of the LTE system: the LTE Advanced (LTE-A) system. Carrier aggregation (CA), where two or more carriers are aggregated, is introduced into the LTE-A system in order to support wider transmission bandwidths, e.g. up to 100 MHz and for spectrum aggregation. Besides, other than a single-NB deployment, i.e. services in a same area are provided by a same Node B, CA may also be applied to a multi-NB deployment, i.e. services in a same area are provided by multiple Node Bs. When CA is applied in the single-NB deployment, since time alignments of all carriers are the same, operations of uplink synchronization are simpler. However, when CA is applied in the multi-NB deployment, different carriers may have different time alignments. In such a situation, if the UE receives the PDCCH order for performing uplink synchronization, since the UE can use multiple carriers at the same time, the UE has to choose a carrier to perform the RA procedure, which reduces transmission efficiency and spectrum aggregation.